1. Field of the Invention
The present invention relates to solar cells. More particularly, the present invention relates to techniques for inexpensive manufacturing and fabrication of high efficiency silicon solar cells using uncomplicated high throughput processes.
2Description of the Prior Art
Semiconductor solar cells are used to convert light energy to electrical energy. Typically, a silicon solar cell consists of a P-N junction. Upon impact of photons in the solar spectrum with the semiconductor, the energy of some of such photons generates electron-hole pairs in the semiconductor. These electron-hole pairs traverse the semiconductor's length and cause a potential difference across the semiconductor and a current in a load connected thereto.
Various techniques for manufacturing and fabricating semiconductive silicon solar cells are known in the prior art. One goal of fabrication of silicon solar cells is to achieve a high efficiency in the conversion of the photon energy to electrical energy. Efficiencies as high as 18% to 20% have been reached in production environments. Another goal of manufacturing silicon solar cells is to reduce the complexity of fabrication, thereby achieving lower costs and higher throughputs. Yet another objective in the production of silicon solar cells is to increase the total energy output while reducing the area of the silicon cell, thereby achieving a lower cost, a lower size and a lower weight. Manifestly, these objectives are often conflicting.
The prior art has attempted to reach some of the aforementioned objectives by utilization of various process techniques. These attempts, however, have not been successful in reaching high efficiency, high throughput and inexpensive solar cells. One reason for this shortcoming is that the prior art has always aimed at perfecting a process step to achieve high efficiency solar cells, usually at the expense of complicating the technology, in turn resulting in expensive and low throughput manufacturing. For example, some prior art techniques have focused on the reduction of electrical interconnect area atop a solar cell in order to reduce the amount of light blocked by such interconnect. Efforts to achieve this goal by "laser punching" (wherein a hole is laser punched in the semiconductor body of a series of adjacent cells and the interconnect is routed through the common hole) have proved to be expensive, and inefficient. As another example, some prior art solar cells have their interconnect on the rear surface of the cell, thus avoiding shading of the front surface. However, the use of masking technology and evaporating metals to form the desired complicated pattern of contacts on the rear surface is both inefficient and expensive.
Another prior art technique to lower the cost of solar cells is to reduce the semiconductor substrate thickness. However, if the thickness of the semiconductor substrate drops below the diffusion length of the minority carriers the recombination of the minority carriers at the rear surface ohmic contact would be increased, in turn resulting in a reduction in the efficiency of the cell. This reduction of efficiency problem was later addressed by creating a potential barrier for the minority charge carriers at the back surface so that the carriers cannot reach the rear surface ohmic contact. This potential barrier was created by diffusion or ion implantation of sufficient impurity atoms in the rear surface of the solar cell. However, the diffusion or ion implantation of impurity atoms in the back surface of the solar cell is complicated, expensive and creates defects at the back surface.
Furthermore, the thinness of solar cells is limited by concerns other than the reduction of efficiency or complicated manufacturing discussed above. Namely, solar cells may get to be too thin to be practical since their brittleness makes it very difficult to attach external electrical contacts on the cells. Moreover, for solar cells that are too thin, some low efficiency still persists due to the poor quality of the thin substrate produced by deposition of the thin semiconductor material on a foreign substance. Thin cells also demonstrate stability problems caused by the chemical reaction of the semiconductor with the ambient (such as oxygen or water).
As a further example, the prior art has utilized very complicated and costly patterning technology to form precise pyramid surface structures to attain "texturized" surfaces in order to reduce the reflectivity of the front surface of solar cells. The improvement in the efficiency of the cells fabricated by this precise texturizing technique may be achieved by less costly methods.
There therefore exists a need for a less complicated, less expensive and higher throughput manufacturing of high efficiency silicon solar cells. The purpose of the present invention is to fulfill this need.